AC/DC single-phase power conversion systems are widely used in industry to convert available electrical energy from the utility mains to a DC output voltage. In various industrial applications such as telecom, network server power supplies, plug-in and hybrid electric vehicles, etc., AC/DC converters are extensively employed to provide an isolated DC voltage from 12V to 600V. For applications in the range of few kilo-watts, AC/DC power conversion systems generally consist of a single phase front-end power factor correction (PFC) converter followed by an isolated full-bridge pulse width modulator (PWM) converter as shown in FIG. 1. The front-end converter must comply with the stringent regulatory standards on the input current harmonics imposed by different agencies. Boost converters are generally used to implement the PFC for the AC/DC converters. In order to effectively minimize the high frequency ripple of the converter input current and output capacitor, multiple boost converters are typically interleaved especially for higher power applications.
FIG. 2 shows the conventional control system of the AC/DC PFC boost converter according to the prior art. In this conventional control system, there is an external voltage loop to regulate the DC-bus voltage and an internal current loop to shape the input current of the converter. There are fundamental challenges in both external voltage loop and internal current loop of the conventional control system. In order to implement the internal current loop, precise information of the boost inductor current is required. There are three different techniques to measure the inductor current. Since the inductor current is low frequency, a Hall effect sensor can be used to measure the inductor current. However, Hall effect sensors suffer from several practical difficulties. Due to the remnant flux, these sensors introduce a time varying DC-bias into the control system. Therefore, a correction algorithm has to be added to compensate for the time varying DC bias. This algorithm increases the complexity of the implementation of the control system. As well, the bandwidth of the Hall effect sensors are limited and they introduce delay into the closed-loop control system. This delay may jeopardise the stability of the control system. Finally, Hall effect sensors are very costly and can significantly contribute to the overall cost of the converter.
The second technique to measure the inductor current is through resistive current sensors. Resistive current sensors require a very precise and noise-free differential amplifier. Also, they increase the power losses of the converter. These power losses are not preferable, especially in higher power applications. These resistive current sensors are also very costly in applications where a very precise current value is required.
The most common method used in industry for determining the inductor current is that of using a Current Transformer (CT) to sense the inductor current. Since the inductor current is low frequency, the CT is usually placed in series with the boost power semiconductor in order to sample the inductor current when the switch is ON. FIG. 3 shows the schematic of the PFC boost converter with CT in series with the power semiconductor. This sensing technique is an affordable solution and is widely used in industrial products. However, the current measurement using a CT creates some major difficulties for the converter. Placing a CT in series with the boost power semiconductor increases the inductive path and causes high voltage spikes across the power semiconductors during the switching transitions. Also, CTs restrict the maximum duty cycle of the converter. This is because the magnetizing inductance of CTs needs to be reset in each switching cycle. Therefore, a suitable amount of time is required to guarantee the reliable performance of CTs. This issue is very pronounced at the zero crossings of the input voltage where the converter should operate with a very high duty ratio.
Another issue related to the use of CTs for sensing current occurs when multiple boost converters are interleaved to share the current and to effectively reduce the input current ripple in higher power applications. FIG. 4 shows a two phase interleaved boost converter. According to FIG. 4, in order to eliminate the switching noise and measure the average inductor current in one switching cycle, the Analogue-to-Digital Converter (ADC) should sample the current at the midpoint of the duty ratio (as shown in FIG. 4). However, due to the phase-shift between the boost phases, the switching noise of one phase may affect the sampling of the next phase. This highly degrades the reliability of the control system. Because of this, in practical circuits a low-pass filter is often used to attenuate the switching noise. Nevertheless, the low-pass filter introduces delay to the control loop and deteriorates the stability margins. Considering all the fundamental issues regarding the current sensors in boost PFC converter, current sensorless techniques are very advantageous in this particular application. Such a sensorless approach can provide a clean and noise-free estimation of the inductor current. As well, it can also provide a very cost-effective solution for this application.
Previous attempts have been made to implement a sensorless current estimators. In one attempt, a single-loop current sensorless control approach was used for a boost PFC converter. For this attempt, a single voltage loop was used to control the converter. While this attempt seems to provide a very simple solution for boost PFC converter, it is very sensitive to the converter parameters. In particular, at light loads the performance of the converter significantly degrades especially close to the current zero crossings.
In another attempt, a modified version of the single-loop current sensorless control technique was proposed. This attempt was able to offer better performance in terms of Total Harmonic Distortion (THD) and zero crossing distortion.
Another sensorless technique was also proposed which was able to determine the duty ratio of the boost PFC converter based on the input AC voltage. The idea behind this proposal was to calculate the output voltage using the input voltage from a voltage sensor and from the voltage drop across the inductor derived based on the converter circuit. Then, the duty ratio is determined using the input and output voltage. However, this approach is only useful for applications where the load variations are very limited and where a high quality input current is not required.
A further attempt has been able to compensate for the error caused by parasitic components. This further attempt demonstrated high quality input current for heavy loads but the current quality deteriorates at light loads.
Considering all the issues related to the current measurement for the boost inductor, eliminating the current sensor is very advantageous for various applications. Eliminating the current sensor can not only improve the performance of the control system but also reduce the overall cost of the converter significantly. However, the particular topology of the boost PFC converter makes the design of the observer very challenging. The topology of the boost converter results in a highly nonlinear mathematical model for the converter. Therefore, a nonlinear observer is required to estimate the inductor current. Also, in many applications, the load information is not available for the observer to be used in the observer structure. Thus, the load value is an unknown parameter for the observer. The other difficulty related to this particular topology is that the converter loses its observability at certain operating conditions.
There is therefore a need for systems, methods, and devices which mitigate if not overcome the shortcomings of the prior art. As well, it would be preferred for any solutions to address the issue of the non-observability of the converter under certain conditions.